multiprocessing Pool (and context manager) fixes/improvements

[OddBloke]

This fixes #1559.

[OddBloke]

The example in #1559:

from multiprocessing import Pool  # Factory function
from multiprocessing import pool  # Submodule


def test(p: Pool) -> None:
    print(type(p))


test(pool.ThreadPool())


try:
    class MyPool(Pool): ...
except:
    print('Cannot subclass')

class MyRealPool(pool.Pool): ...

test(MyRealPool())

now renders:

1559-0.py:5: error: Invalid type "multiprocessing.Pool"
1559-0.py:6: error: No overload variant of "type" matches argument types [Pool?]
1559-0.py:13: error: Invalid type "multiprocessing.Pool"
1559-0.py:13: error: Invalid base class

Once the signature of test is fixed to def test(p: pool.Pool) -> None: as it now should be, we get:

1559-0.py:13: error: Invalid type "multiprocessing.Pool"
1559-0.py:13: error: Invalid base class

which matches what we would expect to happen if we tried to subclass a function/method.

[JelleZijlstra]

Thanks for the detailed writeup!